The invention is directed to a process for the production of the most finely divided oxides of metals and/or silicon through hydrolytic reaction of the volatile chloride of the metal and/or silicon in a flame, in which case the chloride is supplied in admixture with the gases to be burned and air or oxygen to the flame burning out of a burner into a reaction space and brought to reaction with the formation of water and subsequently the oxide aerosol formed together with the residual gas is allowed to pass through a subsequent cooling system and separated off from the residual gas in separating apparatus.
In the production of finely divided oxides of metals and/or of silicon through the hydrolytic reaction of the volatile chloride of the metal and/or of silicon in a flame there is necessarily formed as a byproduct chlorine gas which must be removed from the residual gas of the reaction.
From German OS No. 2533925 (Degussa) it is known that the chlorine formed in the flame reaction is reduced with hydrogen during the cooling off below the reaction temperature of hydrogen with the oxygen contained in the residual gas.
However, this process has the disadvantage that residual gas obtained therewith still contains up to 300 mg/Nm.sup.3 of chlorine. The reduction of this residual chlorine content to a value of &lt;10 mg/Nm.sup.3 of chlorine by absorption with caustic soda solution has been found to be barely possible to carry out and uneconomical because of the presence of carbon dioxide.
Carbon dioxide is formed through the use of organochlorosilanes as starting materials and/or through carbon containing fuel gases. The carbon dioxide formed is absorbed in competition to the elemental chlorine so that lower chlorine contents of the residual gas can only be attained by the addition of large amounts of alkali and several absorption devices connected in succession, such as e.g. wash towers.
The problem of the present invention was to reduce the residual chlorine content of the waste gases to a value of &lt;10 mg/Nm.sup.3 is an economically replaceable manner.